1. Field of the Invention
The present invention relates to a modulation/demodulation circuit for a digital signal recorder/reproducer, and more particularly, to a modulation/demodulation circuit for enhancing the accuracy of restoring a carrier signal, in an apparatus for recording/reproducing with inserting a pilot signal in order to restore the carrier signal.
The instant application is based on Korean Patent Application No. 93-20333, which is incorporated herein by reference for all purposes.
2. Brief Discussion of Related Art
As modern technology undergoes a gradual transformation from analog to digital processing techniques, various kinds of digital recording and reproducing methods are being proposed for various applications. One such application is a digital signal magnetic recorder/reproducer.
Although the digital signal magnetic recorder/reproducer, which records and reproduces a digital image signal, is excellent in terms of picture quality and dubbing performance in comparison with an analog signal magnetic recorder/reproducer, which records and reproduces an analog image signal, the quantity of data which must be recorded on the tape may be over ten times as large as that of a comparable analog signal magnetic recorder/reproducer used in recording the same image signal.
In a conventional record modulation method used for the digital signal magnetic recorder/reproducer, base-band frequency modulation methods such as non-return-to-zero-inverse (NRZI) modulation, partial response (PR) modulation, eight-to-fourteen modulation (EFM), etc., have been employed due to the difficulty in recording and reproducing direct current components. Such base-band frequency modulation methods are performed by converting the zero-run length of the data stream expressed in binary codes, concentrating a frequency spectrum of the signal in the mid-band, and then recording the signal having the concentrated frequency to thereby attain a high-density recording. However, in the base-band frequency modulation method, the signal level to be recorded has only two potential values, i.e., logic "high" and logic "low", which makes high-density recording difficult because of low frequency band utilization efficiency.
Therefore, a channel coding technology suitable for high-density recording is required. A modulation method, which has been used in the communication field, is changed and applied in a form suitable for recording and reproducing, thereby increasing the frequency bands utilization efficiency and improving the recording bit rate without increasing the number of recording channels, is required.
Accordingly, in order to realize a high-density recording, multi-level digital modulation methods such as quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), etc., which are known from other communication fields, have been introduced. As a result, the increase in utilization efficiency of frequency bands permits high-density recording.
FIG. 1 shows a modulation system in a common digital signal recorder/reproducer, wherein an input signal is converted into a digital signal in A/D converter 10 and is quadrature amplitude and phase modulated in QAM modulation circuit 20 before being converted back into an analog signal in D/A converter 30.
The frequency of a bias signal F.sub.B generated by bias signal generator 41 and the maximum frequency f.sub.H of the recording signal band are recorded onto a magnetic recording medium such that the following relation is true. EQU F.sub.B .gtoreq.3f.sub.H . . . (1)
In adder 42, the quadrature amplitude modulated signal, which is output by D/A converter 30, is added to bias signal F.sub.B. Then, the output from adder 42 is recorded onto the magnetic recording medium as a magnetizing signal via record amplifier 50. The bias signal F.sub.B is used for correcting a hysteresis characteristic of the magnetizing signal.
FIG. 2 shows the demodulation system in a common digital signal recorder/reproducer. It will be appreciated that the demodulation process is, essentially the reverse of the modulation process.
Referring to FIG. 2, the modulated signal recorded onto magnetic recording media is reproduced and amplified by a reproduction amplifier 60, is then converted into digital signal in an A/D converter 70, and is applied to a reproduction equalizer 80. In reproduction equalizer 80, signal distortion and the diminished characteristics of the signal which can occur during transmission are corrected. Thereafter, in QAM demodulation circuit 90, the modulated signal output from reproduction equalizer 80 is demodulated and restored to the original digitized signal. Then, the restored signal is converted back into an analog signal in a D/A converter 100 for output.
FIG. 3 is a circuit diagram of the QAM modulation circuit 20 shown in FIG. 1, which includes a mapper 21, wherein the partial bits which are coded and the remaining bits which are not coded, which are output from A/D converter 10, are input together. Then, the input data bits are simultaneously processed in parallel to the degree required to generate a predetermined number of bits so that the coding gain is made large when a decoding operation is subsequently performed, separated into I and Q channel data, and output.
First and second raised cosine filters (RCF) 22 and 26 perform band limiting and waveform shaping in order to remove any inter-symbol interference (ISI).
In a phase-locked loop (PLL) circuit 24, a carrier signal is generated and input to a first balanced modulator 23 directly and to a second balanced modulator 27 via a phase shifter 25, which shifts the output of PLL 24 by 90 degrees. Accordingly, a first carrier signal input to first balanced modulator 23 has a sine component while a second carrier signal input to second balanced modulator 27 has a cosine component.
Then, in first balanced modulator 23, I-channel data from first RCF 22 and the first carrier signal (sine) are multiplied and balance-modulated, while in second balanced modulator 27, Q-channel data from second RCF 26 and the second carrier signal (cosine) are multiplied and balance-modulated. The balance-modulated I- and Q-channel signals are combined in adder 28 and the combined signal is then output.
As shown in FIG. 4, which illustrates the spectral characteristics of the output of the QAM modulation circuit of FIG. 3, the symmetrical upper and lower sidebands of a carrier frequency f.sub.c are produced at the output of adder 28. It will be noted that the carrier signal component has been removed in the f.sub.c band.
FIG. 5 shows the frequency characteristics of first and second raised cosine filters 22, 26 of FIG. 3. Each raised cosine filter consists of a low-pass filter for waveform shaping and band limiting, resulting in a low-pass Nyquist filter characteristic.
On the other hand, when 16 QAM modulation is adapted in the modulation circuit shown in FIG. 3, and a carrier restoring circuit of FIG. 6 is adapted in QAM demodulation circuit 90 shown in FIG. 2 and used to recover a carrier signal, a multiplier 91 requires sixteen squares as compared with common QAM modulation. This is extremely difficult in practice, however, and even if it can be realized, the circuit cost becomes excessive. Even when the carrier signal may be restored by realizing the carrier restoring circuit shown theoretically in FIG. 6, the real time operation is impossible when f.sub.c or an actual frequency of the base-band is high.
To overcome the above problems in FIG. 3, another form of QAM modulation circuit of FIG. 7 is shown, wherein pilot signal generator 129 and adder 130 are connected to the end of adder 128, to thereby transmit the pilot signal together with the modulated signal to a transmission path. While the modulation circuit has poor power efficiency as compared with the modulation circuit shown in FIG. 3, the carrier signal can be easily restored by real time operation when a complementary carrier restoring circuit of the demodulation circuit is used, as shown in FIG. 10.
As shown in FIG. 7, when the pilot signal having the frequency higher than the doubled frequency of the carrier signal is generated by pilot signal generator 129 and inserted in the output signal of adder 128, the amplitude characteristic is poor at the frequency of the upper side-band if the bandwidth is narrower than the signal band transmitted by the channel, although no problems are encountered when the bandwidth is wide enough. As a result, the signal to noise ratio (S/N ratio) is poor. In addition, jitter occurs more often than in case where the carrier signal alone is used. Therefore, restoration of the carrier signal by the receiver is difficult.
FIG. 8 shows a frequency spectrum of the output of the QAM modulation circuit shown in FIG. 7 when a pilot frequency f.sub.p is equal to 2f.sub.c. It will be noted that when pilot frequency f.sub.p is 2f.sub.c, the following two problems occur.
First, as shown in FIG. 8, since pilot frequency f.sub.p is on the uppermost portion of the upper sideband, the amplitude characteristic, i.e. , the S/N ratio, is poor and the pilot signal can be easily interfered with by the adjacent channels.
Second, when the QAM modulation circuit is used for a video tape recorder, jitter occurs much more in pilot frequency f.sub.p than in the low frequency band. As a result, it is difficult to perfectly restore the carrier signal.
FIG. 9 shows the frequency spectrum of the output of the QAM modulation circuit shown in FIG. 7, when pilot frequency f.sub.p is equal to f.sub.c.
When pilot frequency f.sub.p is f.sub.c, the effects of the interference by the adjacent channels and the upper sideband are enhanced. It will be appreciated from the spectrum that the S/N ratio is still not enhanced.
FIG. 10 shows another form of carrier restoring circuit used for the QAM demodulation circuit 90 of FIG. 2, which forms a circuit for restoring the carrier signal from the QAM signal modulated by the QAM modulation circuit shown in FIG. 7. In band-pass filter 191, the band loaded with the pilot signal from the received signal is filtered and the clock signal having the frequency corresponding to carrier signal is restored by PLL 192.
However, in the QAM modulation circuit and the carrier restoring circuit shown in FIGS. 7 and 10, respectively, a bandpass filter (BPF) having a narrow bandwidth has to be used in order to restore only the carrier signal from a frequency band, for implementing a method for transmitting the original signal added with the carrier signal at the carrier location. Also, even if it is possible, restoring the carrier signal is very difficult due to the unnecessary noise (originally, this is signal information which is regarded as noise from the perspective of the carrier signal) of the peripheral portion.
When the QAM modulation circuit shown in FIG. 11 is used in order to solve the above problem, the difference is that burst signal insertion circuits 222 and 227 are used in front of first and second RCFs 223 and 228 in FIG. 11, as compared with the modulation circuit shown in FIG. 3.
FIG. 12 is a waveform diagram of time axis of the output of QAM modulation circuit shown in FIG. 11, while FIG. 13 is a graph showing the output frequency spectrum of QAM modulation circuit.
A method for adding a burst signal and restoring a carrier signal in the modulation circuit shown in FIG. 11, has several problems. First, the space for the information is reduced since the transmission efficiency of the information is poor. Second, the accuracy of restoring a carrier signal is deteriorated since jitter of the carrier signal does not correspond to that of the information throughout the signal sections.